Elasticsearch requires a full cluster restart when upgrading across major
versions. Rolling upgrades are not supported across major versions. Consult
this table to verify that a full cluster restart is
required.

The process to perform an upgrade with a full cluster restart is as follows:

Disable shard allocation

When you shut down a node, the allocation process will immediately try to
replicate the shards that were on that node to other nodes in the cluster,
causing a lot of wasted I/O. This can be avoided by disabling allocation
before shutting down a node:

Shard recovery will be much faster if you stop indexing and issue a
synced-flush request:

POST _flush/synced

A synced flush request is a “best effort” operation. It will fail if there
are any pending indexing operations, but it is safe to reissue the request
multiple times if necessary.

Shutdown and upgrade all nodes

Stop all Elasticsearch services on all nodes in the cluster. Each node can be
upgraded following the same procedure described in Stop and upgrade a single node.

Upgrade any plugins

Elasticsearch plugins must be upgraded when upgrading a node. Use the
elasticsearch-plugin script to install the correct version of any plugins
that you need.

Start the cluster

If you have dedicated master nodes — nodes with node.master set to
true(the default) and node.data set to false — then it is a good idea
to start them first. Wait for them to form a cluster and to elect a master
before proceeding with the data nodes. You can check progress by looking at the
logs.

Use these APIs to check that all nodes have successfully joined the cluster.

Wait for yellow

As soon as each node has joined the cluster, it will start to recover any
primary shards that are stored locally. Initially, the
_cat/health request will report a status of red, meaning
that not all primary shards have been allocated.

Once each node has recovered its local shards, the status will become
yellow, meaning all primary shards have been recovered, but not all replica
shards are allocated. This is to be expected because allocation is still
disabled.

Reenable allocation

Delaying the allocation of replicas until all nodes have joined the cluster
allows the master to allocate replicas to nodes which already have local shard
copies. At this point, with all the nodes in the cluster, it is safe to
reenable shard allocation:

The cluster will now start allocating replica shards to all data nodes. At this
point it is safe to resume indexing and searching, but your cluster will
recover more quickly if you can delay indexing and searching until all shards
have recovered.